> > I'm passing through a virtual PCI device to a QEMU guest via VFIO/mdev > and I > > notice that MSI-X interrupts are disabled in the device (MSIXCAP.MXC.MXE > is > > zero) and the BARs containing the table and PBA (4 and 5 in my case) are > never > > accessed. However, whenever I fire an MSI-X interrupt from the virtual > device > > (although I'm not supposed to do so as they're disabled), the guest seems > to > > correctly receive it. I've started looking at hw/vfio/pci.c and it seems > > that > > VFIO handles MSI-X interrupts there, including masking etc? > > Yes, the vector table and PBA are emulated in QEMU, the latter lazily > only when vectors are masked, iirc. The backing device vector table > should never be directly accessed by the user (it can be, but you can > just discard those accesses), MSI-X is configured via the > VFIO_DEVICE_SET_IRQS ioctl, which configures the eventfd through which > an mdev driver would trigger an MSI. When you say that you "fire and > MSI-X interrupt from the virtual device" does this mean that you're > signaling via one of these eventfds? It looks to me like emulating the > MSI-X enable bit in the MSI-X capability is probably the responsibility > of the mdev vendor driver. With vfio-pci the VFIO_DEVICE_SET_IRQS ioctl > would enable MSI-X on the physical device and the MSI-X capability seen > by the user would reflect that. Are you missing a bit of code that > updates the mdev config space as part of the SET_IRQS ioctl? Thanks,
Indeed I fire interrupts via the eventfd and it works correctly. I just couldn't understand how it could possibly work since the table and PBA BARs were never accessed and the MSI-X enable bit was not set. It makes perfect sense now why it works since QEMU does it all.
